Add full CRUD interface for Showcase management

Implemented complete web interface for managing showcases (display areas for ready-made bouquets) with:

**Backend:**
- ShowcaseForm with validation (unique name per warehouse)
- ShowcaseListView with filtering by warehouse and status
- ShowcaseCreateView, ShowcaseUpdateView with success messages
- ShowcaseDeleteView with active reservation validation
- URL routes: list, create, edit, delete

**Frontend:**
- List page with warehouse grouping, active reservations count
- Responsive table with filters (warehouse, status)
- Create/edit form with Bootstrap styling
- Delete confirmation with active reservations check
- Breadcrumb navigation

**Features:**
✅ One warehouse can have multiple showcases (ForeignKey relationship)
✅ Unique showcase names within each warehouse
✅ Display active reservation counts for each showcase
✅ Prevent deletion if showcase has active reservations
✅ Auto-select default warehouse when creating showcase
✅ Navigation link added to main navbar between "Касса" and "Склад"
✅ Active state highlighting in navigation

**Files created:**
- inventory/forms_showcase.py (ShowcaseForm)
- inventory/views/showcase.py (4 CBV views)
- inventory/templates/inventory/showcase/ (list, form, delete templates)

**Files modified:**
- inventory/urls.py (added showcase routes)
- inventory/forms.py (added Showcase import)
- templates/navbar.html (added "Витрины" link)

URL structure:
/inventory/showcases/ - list
/inventory/showcases/create/ - create
/inventory/showcases/<id>/edit/ - edit
/inventory/showcases/<id>/delete/ - delete

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

myproject/inventory/urls.py

@@ -26,6 +26,8 @@ from .views import (
     # StockMovement
     StockMovementListView,
 )
+# Showcase views
+from .views.showcase import ShowcaseListView, ShowcaseCreateView, ShowcaseUpdateView, ShowcaseDeleteView
 from . import views
 
 app_name = 'inventory'
@@ -95,4 +97,10 @@ urlpatterns = [
 
     # ==================== MOVEMENT (READ ONLY) ====================
     path('movements/', StockMovementListView.as_view(), name='movement-list'),
+
+    # ==================== SHOWCASE ====================
+    path('showcases/', ShowcaseListView.as_view(), name='showcase-list'),
+    path('showcases/create/', ShowcaseCreateView.as_view(), name='showcase-create'),
+    path('showcases/<int:pk>/edit/', ShowcaseUpdateView.as_view(), name='showcase-update'),
+    path('showcases/<int:pk>/delete/', ShowcaseDeleteView.as_view(), name='showcase-delete'),
 ]